What ram upgrade to go for?

So i've decided to join the world of Mac users, i'm getting myself a 20" imac from ebay but i want to upgrade the ram to 4gb (4GB 800MHz DDR2 SDRAM - 2x2GB). who should i get this through and which make is a good way to go?
Dabs.com has loads to choose from but i'm not sure which goes with a Mac?

 

Yeah thanks, i had a feeling that would be the response i go.
I didnt notice the guides until after i had posted this. but they've been a big help. thanks